Thermal energy harvesting optimisation with bistable elements and collaborative behavior

ABSTRACT

System for converting thermal energy into electrical energy (S 1 ) intended to be arranged between a hot source (SC) and a cold source (SF), comprising means for converting thermal energy into mechanical energy ( 6 ) and a piezoelectric material, with the means for converting thermal energy into mechanical energy ( 6 ) comprising groups (G 1,  G 2 ) of at least three bimetallic strips ( 9, 11, 13 ) linked mechanically together by their longitudinal ends and suspended above a substrate ( 12 ), each bimetallic strip ( 9, 11, 13 ) comprising two stable states wherein it has in each of the states a curvature, with two directly adjacent bimetallic strips ( 9, 11, 13 ) having for a given temperature opposite curvatures, with the switching from one stable state of the bimetallic strips ( 9, 11, 13 ) to the other causing the deformation of a piezoelectric material.

TECHNICAL FIELD AND PRIOR ART

This invention provides a system for converting thermal energy intoelectrical energy.

Electronic circuits, when they are operating, produce heat. This heat isnot used and must be removed in order to avoid deteriorating thecircuits. Other sources of heat are also present in our environment,such as for example conduits, exhaust, the walls of industrial machines,etc., of which the heat given off is not used.

It is considered to recover this heat in order to convert it intoelectrical energy.

It is for example considered to use bimetallic strips, the latter areformed of two strips of different metals, materials or alloys withdifferent coefficients of expansion, flexible, welded or glued to oneanother, in the lengthwise direction. Due to the different coefficientsof expansion of the two strips, the bimetallic strip are deformed with alarge amplitude when it is heated as well as when it is cooled. When itis heated, the latter switches from a substantially flat shape to ashape having a certain curvature. The orientation of the curvaturedepends on the temperature to which it is subjected, and the initialproperties of the material (thickness, coefficients of thermalexpansion, etc.). This deformation is converted into electrical energyby a transducer, for example a piezoelectric material which is deformedor shocked when the bimetallic strip becomes curved. The energytransmitted to the piezoelectric material, and therefore the energyrecovered are not optimal.

There are also preformed bimetallic strips that have a first and asecond stable state according to the temperature they are subjected to.In each of the stable states, they have a curvature or deformation, withthe curvatures or deformations of the two stable states being oppositein most cases. These bimetallic strips are also designated as“blistering bimetallic strips”. When such bimetallic strips are heatedand switch from a first stable state to a second stable state, this isreferred to as “blistering”, and when the bimetallic strips are cooled,and switch from the second stable state to the first stable state, thisis referred to as unblistering. During the blistering and theunblistering, a large quantity of energy is released.

DESCRIPTION OF THE INVENTION

The purpose of this invention is to offer a system for convertingthermal energy into electrical energy with improved efficiency.

The purpose mentioned hereinabove is achieved by a system for convertingthermal energy into electrical energy comprising a transducer ofmechanical energy into electrical energy and at least one convertor ofthermal energy into mechanical energy, with the convertor comprising atleast two preformed bimetallic strips linked mechanically in series andhaving opposite and controlled curvatures. The bimetallic strips can bemanufactured from a single piece.

In other terms, preformed bimetallic strips are associated in oppositionin such a way that the blistering of one participates in theunblistering of the other, as such reducing the energy required for theblistering or unblistering of each of the bimetallic strips, as suchincreasing the switching frequency of each of the bimetallic strips.

The bimetallic strips operate in a collaborative manner, which makes itpossible to increase the electrical energy recovered.

The subject-matter of the invention is then a system for convertingthermal energy into electrical energy intended to be arranged between ahot source and a cold source, comprising a substrate, means forconverting thermal energy into mechanical energy and means forconverting mechanical energy into electrical energy, with the means forconverting thermal energy into mechanical energy comprising at least onegroup of at least two bimetallic strips linked mechanically together andsuspended at least partially above the substrate, with each bimetallicstrip comprising two stable states wherein they have in each of thestates a curvature, with directly adjacent bimetallic strips havingopposite curvatures, with the switching from one stable state of thebimetallic strips to the other being adapted to cause an excitation ofthe means for converting mechanical energy into electrical energy and ageneration of electricity.

In an embodiment, the bimetallic strips are linked mechanically togetherby their longitudinal ends in such a way as to form a band.

In another embodiment, the bimetallic strips are linked mechanicallytogether by their lateral ends.

In another embodiment, the bimetallic strips are connected to bimetallicstrips by their longitudinal ends and to bimetallic strips by theirlateral ends in such a way as to form a blanket of bimetallic strips.

Advantageously, the bimetallic strips are made of a single piece. Andmore advantageously, in the case where the system comprises severalgroups of at least two bimetallic strips, said groups are made from asingle piece.

More preferably, the group of bimetallic strips comprises an odd numberof bimetallic strips.

In an embodiment, the means for converting mechanical energy intoelectrical energy comprise a piezoelectric material. The piezoelectricmaterial is then more preferably arranged directly on at least onebimetallic strip.

In another embodiment, the means for converting mechanical energy intoelectrical energy comprise a magnetic material. The magnetic materialcan then be deposited on at least one bimetallic strip, and the systemcomprises electrical contacts on the bimetallic strip in order torecover the electricity generated in the bimetallic strip.

Advantageously, each bimetallic strip is covered by a transducer made ofpiezoelectric material or of magnetic material.

In another embodiment, the means for converting mechanical energy intoelectrical energy is of the capacitive type.

Advantageously, at least one bimetallic strip of the group is, in one ofits stable states, in contact with the substrate improving the thermalcontact.

Another subject-matter of the invention is an assembly for convertingthermal energy into electrical energy comprising a hot source and a coldsource located facing one another and a system for converting accordingto the invention, said system for converting being arranged between thehot source and the cold source. The hot source is for example thermallysupplied by an electronic system.

Another subject-matter of the invention is a method of carrying out asystem for converting according to the invention, comprising the stepsof:

-   -   a) depositing a layer of silicon oxide on a substrate made of        silicon,    -   b) carrying out portions made of Si₃N₄ or SiN on the layer of        oxide,    -   c) thermal growth of the oxide between the portions made of        Si₃N₄,    -   d) carrying out bimetallic strips by depositing a first layer of        metal or of a semi-conductor and of a second layer of metal,        with the two metals having different coefficients of expansion,    -   e) carrying out means for converting mechanical energy into        electrical energy,    -   f) removing the oxide in the discrete zones in such a way as to        form cavities between the substrate and the layers of metal.

The step e) can comprise the depositing of a piezoelectric material onthe layers of metal, which is for example PZT.

The step c) consists for example in a local oxidation of the silicon.

During the step d), the first layer of metal can be polycrystallinesilicon and the second layer of metal can be aluminium.

BRIEF DESCRIPTION OF THE DRAWINGS

This invention shall be better understood using the followingdescription and the drawings wherein:

FIGS. 1A and 1B are views of the sides of an embodiment of a system forconverting according to the invention in two states,

FIG. 2 is a side view of another embodiment of a system for convertingimplementing means for converting mechanical energy into electricalenergy of the magnetic type,

FIGS. 3A to 3F are diagrammatical views of the steps of carrying out thesystem of FIGS. 1A and 1B according to an example of the method ofcarrying out,

FIG. 4A is a side view of an alternative embodiment of a system forconverting wherein the bimetallic strips are in contact with thesubstrate in one of their stable states,

FIG. 4B is a side view diagramming a step of carrying out the system ofFIG. 4A.

DETAILED DESCRIPTION OF PARTICULAR EMBODIMENTS

In the FIGS. 1A and 1B, an embodiment is shown of a system forconverting thermal energy into electrical energy according to theinvention. In the rest of the description, the system S1 for convertingthermal energy into electrical energy shall be designated by “system S1”for the purposes of simplicity.

The system S1 is intended to be arranged between a hot source SC, forexample a surface of an electronic component or of any other source ofheat, and a cold source SF, for example a fin radiator or directly theambient air.

The system S1 extends substantially according to a plane P and comprisesa first surface 2 facing or in contact with the hot source SC and asecond surface 4 facing or in contact with the cold source SF, with thesurfaces 2, 4 being arranged on either side of the plane P. The systemS1 is then subjected to a thermal gradient symbolised by the arrow Fsubstantially perpendicular to its surfaces 2, 4.

The system comprises means for converting thermal energy into mechanicalenergy 6 and means for converting mechanical energy into electricalenergy 8, referred to as a transducer.

The transducer or transducers 8 can be formed, for example, by apiezoelectric material, by means of the capacitive type or by magneticmeans.

In the example shown, the means for converting thermal energy intomechanical energy 6 comprise groups G1, G2 of three preformed andblistering bimetallic strips 9, 11, 13 arranged side by side and linkedmechanically, with directly adjacent bimetallic strips the curvatures ifwhich being opposite, as can be seen in FIGS. 1A and 1B. Each bimetallicstrip 9, 11, 13 comprises two longitudinal ends 9.1, 9.2, 11.1, 11.2,13.1, 13.2. The bimetallic strips are linked mechanically together bytheir longitudinal ends in such a way as to form bands of bimetallicstrips. In the example shown, the bimetallic strips are in directcontact by their longitudinal ends. The groups G1, G2, G3 form bandssuspended above a substrate 12 by their ends, with bimetallic stripsable to be freely deformed under the effect of the temperature.

The substrate 12 comprises hollows 14 above which the bands aresuspended.

Advantageously, the three bimetallic strips 9, 11, 13 are made from asingle piece. More advantageously, all of the groups are made from asingle piece, which simplifies the manufacture of the system andimproves the robustness of the system.

Alternatively, it can be considered, in particular for large-sizesystems, to carry out the bimetallic strips separately and then renderthem integral via welding.

As was indicated hereinabove, a bimetallic strip is formed of two stripsof a different metal or alloy having different coefficients ofexpansion, with the two strips being made integral par rolling, welding,gluing or directly by depositing for example by the direct spraying of asecond material on a first material as shall be described in detail inthe rest of the description, in such a way as to form a monolithicelement. As such when one of the strips expands, the bimetallic stripwill become curved. When the bimetallic strip is heated, it switchesfrom a first configuration to a second configuration, this change isdesignated as “blistering” and, when it cools down, it return to itsfirst configuration, this change is designated as “unblistering”. Morepreferably, each band comprises an odd number of bimetallic strips, withthe bands having a plane of symmetry, which simplifies the manufactureas shall be shown in what follows.

The means for converting thermal energy into mechanical energy cancomprise groups of two bimetallic strips, or of more than threebimetallic strips without leaving the scope of this invention. Thenumber of bimetallic strips per band is selected in such a way that theband does not bend under its own weight. Furthermore, the system extendsmore preferably in a plane and comprises a plurality of bandsdistributed over a surface along lines and according to several parallellines.

Furthermore, the means for converting can comprise any number n ofgroups of bimetallic strips, n being an integer greater than or equalto 1. The number of groups can be selected according to the size of thesystem. In the case of micrometric-size systems a large number of groupscan be carried out.

In the example of FIGS. 1A and 1B, the system comprises transducersformed by a piezoelectric material arranged in such a way as to bedeformed by the blistering and unblistering of the bimetallic strips.

In the example shown, the piezoelectric material is formed by portionsof layers of piezoelectric material directly deposited on the bimetallicstrips. In addition, in the example shown, the piezoelectric material isnot deposited on all of the bimetallic strips. More preferably, such amaterial is deposited on each bimetallic strip in order to increase thequantity of energy recovered. The piezoelectric material can also bepositioned on either side of the bimetallic strip in order to maximisethe production of electrical energy. More preferably, each bimetallicstrip comprises its own transducer.

Contacts (not shown) are present on the piezoelectric material in orderto collect the current produced and are connected either directly to aload, or to a device for storing the electricity produced. Thetransducers are connected in parallel. A system wherein all of thebimetallic strips are covered by a single layer of piezoelectricmaterial does not leave the scope of this invention.

The bimetallic strips have for example a thickness between 0.5 μm and200 μm. A length of a “band of bimetallic strips” can be between 10 μmand a few mm, in the case of an application to electronic components.The number of bimetallic strips per system implemented can be severaltens to several thousands.

The operation of the system for converting S1 shall now be explained.

For example, initially the system is in the state shown in FIG. 1A. Thebimetallic strips 9, 13 are curved on the side of the hot source whilethe bimetallic strip 11 is curved on the side of the cold source SF.

Under the effect of the heat given off by the hot source SC, one of thestrips of each of the bimetallic strips 9 and 13 expands. When thelatter are sufficiently expanded the bimetallic strips 9, 13 blister andtheir curvature is inverted and has the configuration of FIG. 1B.Simultaneously, the bimetallic strip 11 which is on the side of the coldsource SF, tends to be deformed in the inverse configuration. When thetwo bimetallic strips 9, 13 blister, the bimetallic strip 11 due to theblistering of the bimetallic strips 9, 13, is driven. As such thebimetallic strip 11 can unblister before it has stored the requiredenergy, its unblistering is facilitated.

This change in configuration of the bimetallic strips has for effect todirectly deform the piezoelectric material, which causes an appearanceof a charge within the material and therefore the generation of anelectrical current.

As such, the bimetallic strips connected mechanically assist each othermutually, the energy required for a change in the configuration istherefore reduced, which has for effect to increase the switchingfrequency of the bimetallic strips and therefore the number ofdeformation cycles of the piezoelectric material, the electrical energycollected is therefore increased.

It is of course understood that the bimetallic strip 11 can blister orunblister before the bimetallic strips 9 and 13.

The bimetallic strips of the same band or of the same system can havedifferent forms and/or be made with different materials and as suchreact to different temperatures, which can be interesting according tothe configuration of the hot source and that of the cold source.

Advantageously, the bimetallic strips are chosen, more particularly thematerials of the strips of the bimetallic strips, in such a way that thetwo transition temperatures causing the blistering and the unblisteringof the bimetallic strips are close together so that the bimetallicstrips have a blistering/unblistering frequency that is even higher andas such causes the piezoelectric element to vibrate with a highfrequency.

Moreover, the portions of the piezoelectric material that cover thebimetallic strips can be made of different piezoelectric materials.

In the case where the transducer is of the capacitive type, it can beconsidered that each bimetallic strip carries a plate facing a fixedplate, with the two plates being separated by a dielectric medium. Thetwo plates then form a variable capacitor. The change in theconfiguration of each bimetallic strip causes the variation in thecapacity of the capacitors. It can be considered that the bimetallicstrips directly form one of the plates of the capacitors.

FIG. 2 shows an embodiment of a system S2 implementing magnetictransducers. The bimetallic strips are covered with a magnetic material16 that generates a fixed magnetic field. Electrical contacts 18 aredirectly formed at the ends of the bands of bimetallic strips that formelectrical conductors. The change in the configuration of the bimetallicstrips, and therefore the deformation of the electrical conductors,cause the generation of a current according to the Lorentz Force Law.

FIG. 4A shows an alternative embodiment wherein the bimetallic strips 9and 13 are, in a stable state, in direct thermal contact with thesubstrate, and the bimetallic strip 11 is in thermal contact with thesubstrate in the other stable state which improves the responsiveness ofthe system and its output.

FIG. 4B diagrammatically shows a step of the method of the system ofFIG. 4A. This step consists in engraving a layer of oxide 104 on agreater thickness in the zones I and II under the longitudinal ends 9.1,13.2 of the bimetallic strips 9 and 13 are suspended, in such a way asto cause a lowering of the position of the band of bimetallic stripsuntil entering into contact with the substrate.

The embodiments described are not in any case limiting and any otherarrangement can be suitable. An arrangement in the form of a matrix doesnot leave the scope of this invention. Each bimetallic strip, excludingthose located on the edge of the matrix, would then be linkedmechanically to a bimetallic strip at each of its longitudinal ends andat each of its lateral ends.

An example of a method for carrying out a system for convertingaccording to the invention implementing a piezoelectric material inrelation with FIGS. 3A to 3F shall now be described. This method is ofthe type of that implemented in microelectronics and makes it possibleto carry out systems particularly suited for converting thermal energygenerated by electronic systems.

On a substrate 102, for example made of silicon, a layer 104 of SiO₂ isdeposited. The element as such obtained is shown in FIG. 3A.Alternatively, it could be TEOS (Tetraethoxysilane).

During a following step a deposit of Si₃N₄ is carried out on the layerof oxide 104. Alternatively it could be SiN. The portions 106 of Si₃N₄are delimited by lithography and engraving. The element obtained as suchis shown in FIG. 3B.

Then a thermal oxidation of the element of FIG. 3B is carried out, forexample via the method of local oxidation of silicon (LOCOS) which iswell known to those skilled in the art. This method consists in applyingvery high temperatures for example between 700° C. and 1300° C. to theelement arranged in an oxygen-rich atmosphere, which causes an increasein the thickness of the layer of oxide between the zones covered by theportions of Si₃N₄. There is the formation of cups 108 on the surface ofthe element shown in FIG. 3C.

During a following step, a depositing of a layer 110 of a first metal ormetal alloy, for example polycrystalline silicon is carried out and thena depositing of a layer of a second metal or metal alloy 112, forexample aluminium. The two metals have different coefficients of thermalexpansion. Alternatively, the second metal could be Ti, Tin, Cu, Au,FeNi, Ni, W, Pt, Ta, TaN, etc. Then via lithography and engraving withstopping on the oxide, the bimetallic strips 9, 11, 13 are delimited.The element obtained as such is shown in FIG. 3D. The bimetallic stripscarried out as such are directly preformed and have a curvature.Furthermore, they are directly linked mechanically by their longitudinalends since they are made of a single piece.

During a following step, a depositing of a layer 114 of piezoelectricmaterial is carried out, for example of PZT (lead zirconate titanate),or AlN, or ZnO. The portions of piezoelectric material 114 above thebimetallic strips carried out in the preceding step are then delimitedby lithography and engraving. The element obtained as such is shown inFIG. 3E.

During a following step, the oxide is removed partially in such a way asto release the bimetallic strips, thus forming cavities under thebimetallic strips. For this, a resin mask 116 is carried out parlithography, in order to delimit the location of discrete cavities andthen an engraving over time is then carried out for example with dilutedhydrofluoric acid, as such forming the discrete cavities 118. The metallayers then comprise suspended portions forming the bands of bimetallicstrips. The surface of the cavities is determined in such a way that aninteger of bimetallic strips is released. The element obtained as suchis shown in FIG. 3F.

More preferably, the bimetallic strips are released as an odd number,with the strips of bimetallic strips having a plane of symmetry thatfacilitates the manufacture of the system for converting.

Finally, the mask 116 is removed. The element obtained as such is shownin FIG. 1A.

In the case where the means for converting mechanical energy intoelectrical energy are of the magnetic type, it is provided for exampleto deposit a magnetic material instead of the piezoelectric material andto carry out contacts directly on the bimetallic strips.

The system can be of any size, it can have millimetric, micrometric andeven nanometric dimensions to the dimensions of a magnitude of 1 metreto several metres.

The system for converting thermal energy into electrical energy offersimproved output as the frequency of blistering-unblistering of thebimetallic strips is increased, as well as the production ofelectricity.

This system makes it possible for example to make use of the heat givenoff by a surface of a printed circuit chip, by an exhaust pipe of amotor vehicle or by the sun or by any other source of heat.

1-20. (canceled)
 21. A conversion system for converting thermal energyinto electrical energy to be arranged between a hot source and a coldsource, comprising: a substrate; means for converting thermal energyinto mechanical energy; and means for converting mechanical energy intoelectrical energy; the means for converting thermal energy intomechanical energy having at least one group of at least two bimetallicstrips mechanically connected to one another and at least partiallysuspended above the substrate, each bimetallic strip having two stablestates in which it has in each stable state a curvature, the bimetallicstrips being directly adjacent having opposite curvatures, the passagefrom one stable state to the other being adapted to cause excitation ofthe means for converting mechanical energy into electrical energy forelectricity generation.
 22. The conversion system according to claim 21,wherein the bimetallic strips are mechanically interconnected by theirlongitudinal ends so as to form a band.
 23. The conversion systemaccording to claim 21, wherein the bimetallic strips are mechanicallyinterconnected by their lateral ends.
 24. The conversion systemaccording to claim 21, wherein the bimetallic strips are connected totheir longitudinal ends by bimetal and the bimetallic strips with theirlateral ends so as to form a bimetallic sheet.
 25. The conversion systemaccording to claim 21, wherein the bimetallic strips are integrallyformed.
 26. The conversion system according to claim 21, wherein thebimetallic strips comprise a plurality of groups for at least twobimetallic strips, said plurality of groups being integrally formed. 27.The conversion system according to claim 26, wherein each groupcomprises an odd number of bimetallic strips.
 28. The conversion systemaccording to claim 21, wherein the means for converting mechanicalenergy into electrical energy comprises a piezoelectric material. 29.The conversion system according to claim 28, wherein the piezoelectricmaterial is disposed directly on at least one bimetallic strip, andfurther comprising electrical contacts on the piezoelectric material.30. The conversion system according to claim 21, wherein each bimetallicstrip is covered by a piezoelectric transducer material.
 31. Theconversion system according to claim 21, wherein the means forconverting mechanical energy into electrical energy comprises a magneticmaterial.
 32. The conversion system according to claim 31, wherein themagnetic material is deposited on at least one bimetallic strip, andfurther comprising electrical contacts coupled to the bimetallic strip.33. The conversion system according to claim 21, wherein the means forconverting mechanical energy into electrical energy are of a capacitivetype.
 34. The conversion system according to claim 21, wherein at leastone bimetallic strip when in one of its stable states is in contact withthe substrate.
 35. A thermal energy assembly comprising: a hot sourceand a cold source; and a conversion system for converting thermal energyinto electrical energy arranged between the hot source and the coldsource, comprising a substrate, means for converting thermal energy intomechanical energy, and means for converting mechanical energy intoelectrical energy, the means for converting thermal energy intomechanical energy having at least one group of at least two bimetallicstrips mechanically connected to one another and at least partiallysuspended above the substrate, each bimetallic strip having two stablestates in which it has in each stable state a curvature, the bimetallicstrips being directly adjacent having opposite curvatures, the passagefrom one stable state to the other being adapted to cause excitation ofthe means for converting mechanical energy into electrical energy forelectricity generation.
 36. The thermal energy assembly according toclaim 35, wherein the hot source is thermally supplied by an electronicsystem.
 37. A method of making a conversion system for convertingthermal energy into electrical energy to be arranged between a hotsource and a cold source and comprising a substrate, means forconverting thermal energy into mechanical energy, and means forconverting mechanical energy into electrical energy; the means forconverting thermal energy into mechanical energy having at least onegroup of at least two bimetallic strips mechanically connected to oneanother and at least partially suspended above the substrate, eachbimetallic strip having two stable states in which it has in each stablestate a curvature, the bimetallic strips being directly adjacent havingopposite curvatures, the passage from one stable state to the otherbeing adapted to cause excitation of the means for converting mechanicalenergy into electrical energy, the method comprising: a) depositing asilicon oxide layer on a silicon substrate; b) forming portions of Si₃N₄or SiN on the silicon oxide layer; c) performing thermal oxide growthbetween the portions; d) forming the bimetallic strips by depositing afirst layer of metal or semiconductor and a second metal layer thereon,the two metals having different coefficients of expansion; e) formingthe means for converting mechanical energy into electrical energy; andf) removing of the oxide in discrete zones so as to form cavitiesbetween the substrate and the metal layers.
 38. The method according toclaim 37, wherein step e) includes depositing a piezoelectric materiallayer on the metal layers.
 39. The method according to claim 37, whereinstep c) is performed by a local oxidation of silicon.
 40. The methodaccording to claim 37, wherein in step d), the first metal layercomprises polycrystalline silicon and the second metal layer comprisesaluminum.